Fuel injection into the cylinders of an internal combustion engine is most commonly achieved using fuel injectors. A commonly used injector is a closed nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifices for resisting blow back of exhaust gas into the injector while allowing fuel to be injected into the cylinder. The nozzle valve also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust.
Recent and upcoming legislation resulting from a concern to improve fuel economy and reduce emissions continues to place strict emissions standards on engine manufacturers. In order for new engines to meet these standards, it is necessary to produce fuel injection systems capable of achieving higher injection pressures, controlled injection rates and fast response while maintaining accurate and reliable control of fuel metering and injection timing functions. As a result, closed-nozzle injectors are undergoing structural modifications which better enable the injectors to produce and withstand the higher injection pressures. However, these improvements often undesirably increase the size of the injector which must conform to overall size restrictions or packaging constraints dictated by the mounting arrangement on a particular engine. Also, as injection pressures increase, greater forces must be applied to the injector components of the injector body to achieve the required sealing at the component interfaces/joints. However, injectors often include internal components which are sensitive to the load applied to the component.
For example, one way of achieving the goal of high pressure and controlled injection metering and timing is to incorporate an injection control valve in the injector body for controlling timing and quantity of injection. These injectors may be "unit" injectors which conventionally include a positive displacement plunger driven by a cam mounted on an engine drive camshaft. U.S. Pat. No. 4,482,094 to Knape and U.S. Pat. No. 4,741,478 to Teerman et al. both disclose closed-nozzle, electromagnetic unit fuel injectors including a single, cam-operated injector plunger and a solenoid controlled valve for controlling the beginning and end of injection and thus, the timing and quantity of fuel injected during each cycle of plunger movement. The solenoid controlled valve operates to allow fuel to flow into and out of a pumping/metering chamber of the unit injector when open but traps fuel in the chamber when closed to cause the injector plunger to force fuel through the injector nozzle into an associated combustion chamber of the engine. With this construction, the solenoid controlled valve is normally biased into an open position to allow excess fuel to be discharged from the pumping/metering chamber to a drain passage while the nozzle valve is biased into a closed position. Upon movement of the solenoid operated valve to a closed condition, a sufficient pressure will build up so as to displace the nozzle valve and allow the injection of fuel to commence.
Although the injectors of Knape and Teerman et al. function to provide sufficient fuel injection capability, both injectors subject the injection control valve components to excessively high axial forces. The internal components of the injector including the actuator, e.g. center tube or control valve cage, are compressively held together between an outer retainer and an upper barrel. In addition, clamping forces, applied to the injector to secure the injector to the cylinder head of an engine, also may act on the control valve assembly. In addition, the fuel pressure in the pumping chamber, created by the cam-operated plunger, imparts a hydraulic force on the control valve assembly. As a result, the control valve components and housing must be formed of sufficient size and strength to withstand such forces, or risk distortion and damage, and thus possible valve malfunction. Consequently, these existing assemblies are unnecessarily large, robust and expensive.
The Knape and Teerman injectors also suffer from other disadvantages. For example, in the Knape design, the coil of the solenoid is arranged concentrically around the injector plunger. Therefore, the solenoid coil inner diameter is determined by the diameter of the injector plunger. As a result, the solenoid coil and fuel injector body have an unnecessarily large diameter. This design results in the loss of space available to the engine intake and exhaust valves. In addition, the armature/control valve arrangement utilizes the magnetic lines of force of the outer pole of the stator positioned beyond the outer radial extent of the coil. Since the armature must be positioned closely adjacent to these outer poles to generate the force requirements of the valve, the armature is required to be larger than the outside diameter of the coil. This large armature mass increases the effects of inertia thereby undesirably increasing response time. The solenoid actuator of the Teerman et al. injector is positioned axially between the injector plunger and the normally closed nozzle tip valve. However, although decreasing the outer diameter of the injector body, this axial arrangement created a fuel injector body having an undesirably large axial length. In addition, the Teerman et al. injector includes a solenoid actuator which utilizes the magnetic lines of force adjacent the outer pole requiring a relatively large armature thereby causing a decrease in response time.
Another disadvantage of the prior art discussed above is that the valve seat of the solenoid operated control valve is positioned a relatively large distance from the pumping/metering chamber. This arrangement increases the length of the fuel transfer passages thereby increasing the compressed fuel volume and, consequently, the response time.
U.S. Pat. No. 5,082,180 to Kubo et al. discloses a closed nozzle injector having an electromagnetic injection control valve which includes a tubular valve member and armature slidably mounted on a guide or center tube member wherein the armature, a stator and an armature biasing spring are located between the pumping chamber and nozzle assembly. The components of the nozzle assembly are held in compressive abutment by threadably connecting an auxiliary retainer to a spring holder to securely engage a main retainer. The control valve assembly is securely positioned in the main retainer by threadably connecting the main retainer to the injector body. However, the threaded connection between the main retainer and the injector body may shift or become loose due to the load on the main retainer transitioning between tension and compression as the plunger reciprocates. As a result, an unnecessarily large amount of axial loading may be imparted to the center tube. This large axial load causes the center tube to dilate and shorten resulting in binding of the valve element and possible shortening of the valve stroke. If the diameter of the center tube is increased to withstand increased loading, the increased size of the center tube/valve element interface allows the injection pressure to cause dilation of the valve element thereby adversely affecting valve movement and seating. Also, this two-piece retainer design unnecessarily results in an excessive number of injector parts. Moreover, the armature sleeve and biasing spring are positioned in a non-overlapping manner with the solenoid stator along the axis of the injector. Thus, the injector in Kubo et al. has an undesirably large axial length which increases response time and high pressure capability due to the length of the high pressure passages. Also, proper alignment and sealing is required at both ends of the center tube and is difficult to achieve due to differences in manufacturing tolerances.
Consequently, there is a need for a compact, inexpensive unit fuel injector having an injection control valve mounted in the injector body which avoids high axial loading of certain internal components, such as the injection control valve, caused by, for example, fuel pressure forces and mounting forces.